A) Field of the Invention
The present invention relates to wet etching for selectively and anisotropically etching a silicon substrate with alkali etchant.
B) Description of the Related Art
It is known for wet etching of this kind that a lamination layer of a silicon oxide film and a silicon nitride film (hereinafter called a silicon nitride/silicon oxide lamination layer) is used as an etching mask in order to prevent permeation of alkali etchant (for example, refer to Japanese Patent Laid-open Publication No. 2000-114248).
According to the studies by the present inventor, it has been found that when wet etching is performed by forming a rectangular mask opening through a silicon nitride/silicon oxide lamination layer, the etched silicon region is not rectangular or cracks are formed in the silicon nitride/silicon oxide lamination layer, in some cases.
FIGS. 9 and 10 illustrate the main processes of silicon wet etching in association with the inventor's studies.
In the process shown in FIG. 9, an etching mask 5 is formed on one principal surface of a silicon substrate 1 made of single crystal silicon. An etching stopper film 2 is formed on the other principal surface of the substrate 1. In forming the etching mask 5, after a silicon oxide film 3 is formed on the substrate surface by thermal oxidation, a silicon nitride film 4 is deposited on the silicon oxide film 3 by chemical vapor deposition (CVD). The silicon nitride/silicon oxide lamination layer of the films 3 and 4 are selectively dry etched to form a rectangular mask opening 4A to form an etching mask 5 made of the left silicon nitride/silicon oxide lamination layer. For example, the etching stopper film 2 is made of a silicone oxide film formed by thermal oxidation. If the etching stopper film is used as a membrane or the like, layers of various configurations can be used, such as depositing a silicon nitride layer on the silicon oxide film by CVD.
In the process shown in FIG. 10, by using the etching mask 5 having the mask opening 4A, the substrate 1 is selectively and anisotropically etched to the surface of the etching stopper film 2 with alkali etchant such as tetra-methyl-ammonium-hydroxide (TMAH) to form a substrate opening 1A.
FIG. 11 is a diagram of the substrate shown in FIG. 10 as viewed from the upper side, the cross sectional view of FIG. 10 being taken along line X-X′ shown in FIG. 11. The inner wall 5a of the mask opening 4A is curved in an inner convex shape due to film stress of the etching mask 5 or the inner wall 1a of a substrate opening 1A is curved in an inner convex shape due to film stress applied to the substrate 1, to cause a shape abnormality B. When the film stress is large, a crack is formed in the etching mask 5 (particularly the silicon nitride film 4) to cause a crack abnormality A.